The invention relates generally to wind turbines, and, in particular, to a system and method for reducing the load imbalance seen by the turbine components (rotor, drive train, tower) during normal operation.
Wind turbines are regarded as environmentally friendly and relatively inexpensive alternative sources of energy. A wind turbine generator generally includes a wind rotor having a plurality of blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive a rotor of an electrical generator to produce electrical power. In modern wind power generation systems, power output from a plurality wind turbine generators, comprising a “wind farm”, is typically combined for transmission to a grid.
Power output of a wind turbine generator generally increases with wind speed until a rated power output is reached. Thereafter, the power output is usually maintained constant at the rated value even with an increase in wind speed. This is generally achieved by regulating the pitching action of the blades in response to an increase in wind speed. With increase in wind speed beyond the rated power output, the blades generally are pitched toward feather (i.e., twisted to be more closely aligned with the direction of the wind), thereby controlling the angular speed of the rotor. As a result, generator speed, and consequently, generator output may be maintained relatively constant with increasing wind velocities.
In case of sudden turbulent gusts, wind speed, wind turbulence, and wind shear may change drastically in a relatively small interval of time. Reducing rotor imbalance while maintaining the power output of the wind turbine generator constant during such sudden turbulent gusts calls for relatively rapid changes of the pitch angle of the blades. However, there is typically a time lag between the occurrence of a turbulent gust and the actual pitching of the blades based upon dynamics of the pitch control actuator and the inertia of the mechanical components. As a result, load imbalances and generator speed, and hence oscillations in the turbine components as well as power, may increase considerably during such turbulent gusts, and may reduce the life of the machine and exceed the maximum prescribed power output level (also known as overspeed limit) causing the generator to trip, and in certain cases, the wind turbine to shut down. The overspeed limit is generally a protective function for the particular wind turbine generator and is based upon fatigue considerations of the mechanical components, such as the tower, drive train, and so forth. Moreover, sudden turbulent gusts may also significantly increase tower fore-aft and side-to-side bending moments due to increase in the effect of wind shear.
Reduction of loading has heretofore been addressed only as pitching the wind turbine blades while taking into account the upwind wind speed measurement, to alleviate the impact of turbulent gust winds on the turbine. Consequently, rotor imbalance due to wind shear and turbulence has only been addressed through pitching of wind turbine blades in a reactive manner, based on tower loading due to wind inflow.
Accordingly, there exists a need for a proactive mechanism to control pitching of the blades of a wind turbine to compensate for rotor imbalance during normal operation by pitching the blades individually or asymmetrically, based not only on the wind speed, but also on the wind turbulence and wind sheer dynamics in front of the rotor, determined before it reaches the rotor.